a) Field of the Invention
This invention relates to processes for the manufacture of electrophoretic displays, in particular, multi-color and sectional electrophoretic displays.
b) Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates. When a voltage difference is imposed between the two electrode plates, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26 (8):1148-1152 (1979)), there are partitions between the two electrode plates for dividing the space into smaller cells, in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered, and in electrical contact, with transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170), the content of which is incorporated herein by reference in its entirety. The improved EPD cells are prepared by, for example, microembossing a layer of a thermoplastic or thermoset precursor composition coated on a first substrate to form microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and top-sealed with a polymeric sealing layer. A second substrate is laminated over the filled and sealed microcups, preferably with an adhesive layer.
The microcup-based multi-color electrophoretic display has many advantages. For example, when the microcup dimensions are formed to match the size of the sub-pixels intended for the red, green and blue color on a thin film transistor backplane, they can be filled individually with the red, green and blue colored electrophoretic fluids to correspond to the geometric arrangements of the sub-pixels on the TFT backplane. This feature allows the possibility of true multi-color displays with active matrix driving. Examples of the driving schemes for the multi-color microcup-based electrophoretic displays are described in detail in U.S. Pat. No. 6,885,495 (corresponding to WO03/009059) and U.S. Pat. No. 7,046,228 (corresponding to WO03/016993), the contents of both of which are incorporated herein by reference in their entirety.
From the manufacture point of view, accurate placement of a minute amount of electrophoretic fluids into the designated microcups is theoretically achievable with current technologies. Inkjet printing has been considered as a possible candidate for this task because of its ability to precisely deliver a predetermined volume of a fluid in the form of tiny droplets of a well-controlled size. The tall partition walls of the microcup-based electrophoretic displays appear to also provide a good mechanism to prevent splash and mixing of inkjet-printed electrophoretic fluids. However, there are several very challenging processing issues to be solved before the roll-to-roll manufacturing of multi-color electrophoretic displays can be implemented.
First, the diameter of the charged pigment particles in the electrophoretic fluids is typically in the range of tenths of a micron to several microns. Plugging of the inkjet head nozzles could be a major reliability problem, particularly if the pigment particles have a tendency to flocculate or aggregate during use or storage.
In addition, when a dielectric solvent of low surface tension, such as a perfluorinated or hydrocarbon solvent, is used in the electrophoretic fluids, there are additional difficulties because of the low surface tension of the solvent. The preferred surface tension of conventional inkjet inks or fluids is typically in the range of 30˜45 dyne/cm. The extremely low surface tension (in the range of 14˜30 dyne/cm) of dielectric solvents, particularly the perfluorinated solvents, does present a major problem in the control of the droplet breakdown process. For example, when a perfluorinated solvent is used in an electrophoretic fluid, it is difficult to maintain a negative surface pressure to keep the electrophoretic fluid inside the nozzle. Furthermore, undesirable drying and particle deposition on the nozzle head would occur because of the capillary effect that encourages an outward material flow to the nozzle surface. Therefore, it has been very difficult to manufacture electrophoretic displays with high efficiency and reliability when inkjet printing is involved in the filling of the electrophoretic fluids, particularly when a dielectric solvent of low surface tension is used.
Also, in the roll-to-roll manufacturing process for microcup-based electrophoretic displays, the electrophoretic fluid filling process is immediately followed by a sealing process. When the microcups in one particular area on the web are filled with slightly more fluids than necessary, there is the possibility of “overflow” of the fluids in this area to the adjacent microcups before or during the subsequent sealing step. The “overflow” of an electrophoretic fluid of one color to adjacent microcups containing electrophoretic fluids of other colors means intermixing of different colors, which inevitably would result in reduction of the color purity of the display manufactured therefrom. Therefore, there has been a strong need for a roll-to-roll manufacturing process for multi-color electrophoretic displays with a wider process window to ensure product quality.